First code and output:
NSString *text = #"Sunny";
__weak NSString *string0 = text.lowercaseString;
__weak NSString *string1;
string1 = text.lowercaseString;
NSLog(#"%#, %#", string0, string1);
Output:
(null), sunny
But after I move the declaration of string1 above the text, output is different. Here is the code:
__weak NSString *string1;
NSString *text = #"Sunny";
__weak NSString *string0 = text.lowercaseString;
string1 = text.lowercaseString;
NSLog(#"%#, %#", string0, string1);
Output:
sunny, sunny
I am very confused with the different output:
Why string0 and string1 is different in the first case?
Why the output of second case is different with the first?
Trying to figure out exactly when an object is released or a weak reference nulled can be challenging, and often doesn't really help understanding. Here are some reasons why you can see different results than you expect:
NSString: It is best never to use this type when doing these kind of investigations. String literals are immortal, they are not collected, and you may have a string literal even when you don't expect one.
The auto-release pool: The auto-release pool is really a hangover from the pre-ARC days, but it still exists and many methods return auto-released objects. What this means is many objects will live longer than you might expect, but not too long. However ARC has tricks and can remove objects from the auto-release pool early, so you might first think the object will live longer and then it doesn't...
weak references: After the first two bullets you should guess that as you might have no real idea when an object gets released, if at all, then you might have no real idea when a weak reference gets nulled. Just think "soon enough".
Optimisation: There is some leeway in optimisations the compiler can do which, while retaining the correct semantics of your program, may alter the lifetime of objects.
If you do want to run these kind of investigations then you will probably get further if (a) use your own class types, not anything from the libraries and (b) use #autoreleasepool { ... } blocks to limit the lifetimes of auto-released objects.
As an example, when I ran your code on the compiler I was using I did not get a (null), however changing the first assignment to string0 = text.lowercaseString.mutableCopy did produce one... Figuring out why is left as an exercise...
Have an inquiring mind and explore, that is good, but be prepared for the non-obvious!
HTH
Related
I would like to get a clarification/difference on the NSString declaration. Consider the following codes:
NSString *str = #"string";
NSString *str = [NSString stringWithFormat:#"string"];
NSString *str = [[NSString alloc] initWithString:#"string"];
Can anyone help me to understand the difference between the above three type of string declaration? Does the difference come in terms of memory or will there be any other reasons? I have gone through various posts to understand the difference, but I couldn't understand the exact difference.
Sree
Edit (thanks for the comments):
Using ARC, the first statement is used by the compiler to create a string that is accessible during the lifetime of the app and never deallocated.
The last two statements produce the same kind of string.
Using manual memory management, the second statement produces an autoreleased string.
The last produces a retained string. This means, when using the last statement, you would have to add a release later in the code.
for: NSString *str = #"string"; used when you use as a static string .
for Exmple.
int abc=5;
for: NSString *str = [NSString stringWithFormat:#"%d",abc]; used when you convert your integer or float into string.
for: NSString *str = [[NSString alloc] initWithString:#"string"]; used when above same same reason but only difference is you alloc string and then passes static string.
but in ARC no need to alloc string.
as I remember in this case
NSString *str = #"string";
the memory for the string will be allocated once and will be reused for all same strings in whole application. Something like global constant.
https://en.wikipedia.org/wiki/String_literal
There is the proof: "Objective-C string constant is created at compile time and exists throughout your program’s execution. The compiler makes such object constants unique on a per-module basis, and they’re never deallocated"
https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/Strings/Articles/CreatingStrings.html
The others 2 is the same, also you can formate string with stringWithFormat method: https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/Strings/Articles/FormatStrings.html#//apple_ref/doc/uid/20000943
NSString literal
#"string"
This will make the compiler emit a statically allocated NSString instance. The object itself will never be deallocated (release is a no-op).
Unique instance from format
[NSString stringWithFormat:#"string"]
Here the string is created from parsing and transforming the format string. This involves runtime overhead and returns an autoreleased instance.
Initialize by copying a literal
[[NSString alloc] initWithString:#"string"]
This will (1) create the literal, (2) allocate a default string, (3) throw away the default string, (4) call copy on the literal, and (5) return the result (again, the literal, because copy just returns it). The logic retain count is +1 (automatically handled by ARC).
Transitioning to ARC on iOS.
I have an autoreleased NSString that I use to generate a UTF-8 representation, and rely on pool lifetime to keep the UTF-8 pointer alive:
char *GetStringBuffer(something)
{
NSString *ns = [NSString stringWithSomething:something];
return [ns UTF8String];
}
The nature of something is not important here.
Pre-ARC rules make sure the returned data pointer will stay valid for the lifetime of current autorelease pool. Crucially, I don't carry the NSString pointer around.
Now, under ARC, won't the string be released when the function returns? I don't think ARC will consider a char * to a structure deep inside an NSString a strong reference, especially seeing that it's not explicitly freed ever.
What's the best ARC idiom here?
If you want to guarantee that the return value of UTF8String is valid until the current autorelease pool is drained, you have two options:
Define GetStringBuffer in a file that is compiled with ARC disabled. If stringWithSomething: follows convention, it must return an autoreleased NSString to a non-ARC caller. If it doesn't (e.g. it acts like -[NSArray objectAtIndex:]), you can explicitly retain and autorelease it.
Use toll-free bridging and CFAutorelease:
char *GetStringBuffer(something) {
NSString *ns = [NSString stringWithSomething:something];
CFAutorelease(CFBridgingRetain(ns));
return [ns UTF8String];
}
(I can't delete this because it's accepted, but this answer is incorrect. See Rob Mayoff's answer, and the comments on that answer for an explanation. There is no promise that this pointer is valid past the return statement.)
Rewriting my whole answer. Had to dig and dig, but I believe this is surprisingly safe today (due to improvements in ARC since I had my crashes; I knew something like that was rolling around in the back of my head). Here's why:
#property (readonly) __strong const char *UTF8String NS_RETURNS_INNER_POINTER; // Convenience to return null-terminated UTF8 representation
UTF8String is marked NS_RETURNS_INNER_POINTER, which is really objc_returns_inner_pointer. Calling that method:
the object’s lifetime will be extended until at least the earliest of:
the last use of the returned pointer, or any pointer derived from it, in the calling function or
the autorelease pool is restored to a previous state.
Which is pretty much what you wanted it to do.
In my iOS app I have following code:
case SASpeechSubCase03:
{
SAActivity currentActivity = self.mediator.selectedActivity;
NSString *sActivity = NSLocalizedString(#"activity", #"activity");
NSString *sActivity2 = NSLocalizedString(#"another activity", #"another activity");
if(currentActivity == SAActivityWalk)
{
sActivity = NSLocalizedString(#"walk", #"walk");
sActivity2 = NSLocalizedString(#"walking", #"walking");
}
else
{
sActivity = NSLocalizedString(#"run", #"run");
sActivity2 = NSLocalizedString(#"jogging", #"jogging");
}
return [NSString stringWithFormat:speech.text, sActivity, sActivity2];
break;
}
When I run bots on it, it gave me following warning:
Bot Issue: analyzerWarning. Dead store.
Issue: Value stored to 'sActivity' during its initialization is never read.
File: SAAnnouncementService.m.
Integration Number: 42.
Description: Value stored to 'sActivity' during its initialization is never read.
Bot Issue: analyzerWarning. Dead store.
Issue: Value stored to 'sActivity2' during its initialization is never read.
File: SAAnnouncementService.m.
Integration Number: 42.
Description: Value stored to 'sActivity2' during its initialization is never read.
Can someone tell what the problem might be here?
Any kind of help is highly appreciated!
The problem is that you initialized the variables and then directly started the if-else blocks, without using, i.e. reading, the initial values.
When execution gets to the if-else blocks, it will definitely be assigned a new value, no matter what value it was before.
With the following line :
NSString *sActivity = NSLocalizedString(#"activity", #"activity");
NSString *sActivity2 = NSLocalizedString(#"another activity", #"another activity");
You are assigning string values to the sActivity and sActivity2 objects.
Then, these two values are modified in either if or else statement.
But, as the static analyzer mentions, the initial values of these objects (#"activity" and #"another activity") were never read before the second assignment (in if / else statement).
To avoid this warning you can replace the two lines above, by :
NSString *sActivity = nil;
NSString *sActivity2 = nil;
Hope that helps ;)
When you get a warning, the compiler tells you "what you are doing here looks like nonsense, and is most likely not what you want".
Look at these two statements:
NSString *sActivity = NSLocalizedString(#"activity", #"activity");
NSString *sActivity2 = NSLocalizedString(#"another activity", #"another activity");
Does the assignment serve any purpose? It doesn't look like it. So the compiler thinks "either the guy made a rather expensive call that is completely pointless, or he actually intended to use the result of NSLocalizedString but stored it in the wrong place. "
Since the compiler assumes that people don't do pointless things, it assumes that there is a bug in your code and tells you about it. It's the kind of thing where a human reviewing your code would stop and ask you what you were intending to do there.
In your codes, sActivity would be set to either walk or run within IF/ELSE, so that the value set for sActivity this line
NSString *sActivity = NSLocalizedString(#"activity", #"activity");
would never be read. It might not cause error but analyzer reminded you about this superfluous initialization. Try NSString *sActivity=nil;, see if the warning could be turned down.
You are not using sActivity in if-else blocks, you are simply assigning it values based on decision, So either take it nil string like
sActivity = nil;
or like
NSString *sActivity;
to remove waring .
I found a strange behavior with NSString. I tried to run the below code and noticed this.
NSString *str = [[NSString alloc] initwithstring : #"hello"];
[str release];
NSLog(#" Print the value : %#", str);
Here, in the third line app should crash because we are accessing an object which is released. But it is printing the value of str. It is not crashing. But with NSArray i observed different behavior.
NSArray *array = [[NSArray alloc] initwithobjects : #"1", #"2", nil];
[array release];
NSLog(#"Print : %#", [array objectatindex : 0]);
NSLog(#"Print : %#", [array objectatindex : 0]);
The code has two NSLog statements used for NSArray. Here after releasing when the first NSLog is executed, it is printing value. But when second NSLog is executed, app crashes. App crash is acceptable because the array accessed was released already. But it should crash when the first NSLog is executed. Not the second one.
Help me with this behaviors. How release works in these cases.
Thanks
Jithen
The first example doesn't crash because string literals are never released. The code is really:
NSString *str = #"hello";
[str release];
People get burned with string literals on memory management and mistakenly using == to compare them instead of isEqualToString:. The compiler does some optimizations that lead to misleading results.
Update:
The following code proves my point:
NSString *literal = #"foo";
NSString *second = [NSString stringWithString:literal];
NSString *third = [NSString stringWithString:#"foo"]; // <-- this gives a compiler warning for being redundant
NSLog(#"literal = %p", literal);
NSLog(#"second = %p", second);
NSLog(#"third = %p", third);
This code gives the following output:
2013-02-28 22:03:35.663 SelCast[85617:11303] literal = 0x359c
2013-02-28 22:03:35.666 SelCast[85617:11303] second = 0x359c
2013-02-28 22:03:35.668 SelCast[85617:11303] third = 0x359c
Notice that all three variable point to the same memory.
Your second example crashes at the second NSLog because at the first log, the memory where array was hasn't been re-used, but that first log causes enough activity on the heap to cause the memory to become used by something else. Then, when you try to access it again, you get a crash.
Whenever an object is deallocated and its memory marked as free, there is going to be some period of time where that memory still stores what's left of that object. During this time you can still call methods on such objects and so forth, without crashing. This time is extremely short, and if you're running a lot of threads it may not even be enough to get your method call in. So clearly, don't rely on this implementation detail for any behavior.
As others have said, regarding your first question, NSString literals aren't going to be deallocated. This is true for some other Foundation classes (NSNumber comes to mind) but is an implementation detail as well. If you need to do experiments on memory management, use an NSObject instance instead, as it will not show the unusual behaviors.
When you send a release message on an object, the object is actually not being removed from the memory. The release message simply decrements the reference count by one only. If the reference count is zero the object is marked as free. Then the system remove it from the memory. Until this deallocation happens you can access your object. Even if you release the object your object pointer still points to the object unless you are assigning nil to the pointer.
The first example doesn't crash because string literals are never released. Where the second totally depends on release and retain counter.
Read this article. Its contains short-and-sweet explanation for your query
You Should read this apple guideline
You seem to assume that release should destroy the object immediately. I don't think that's the guarantee that the language makes. What release means is: I have finished using this object and I promise not to use it again. From that point onwards it's up to the system to decide when to actually deallocate the memory.
Any behaviour you see beyond that is not defined and may change from one version of the Objective C runtime to the next.
That's to say that the other answers that suggest the difference is string literals and re-use of memory are currently correct but assuming that the behaviour will always be like this would likely be a mistake.
I've written the following code:
NSString *string = [[NSString alloc] initWithFormat:#"test"];
[string release];
NSLog(#"string lenght = %d", [string length]);
//Why I don't get EXC_BAD_ACCESS at this point?
I should, it should be released. The retainCount should be 0 after last release, so why is it not?
P.S.
I am using latest XCode.
Update:
NSString *string = [[NSString alloc] initWithFormat:#"test"];
NSLog(#"retainCount before = %d", [string retainCount]);// => 1
[string release];
NSLog(#"retainCount after = %d", [string retainCount]);// => 1 Why!?
In this case, the frameworks are likely returning the literal #"test" from NSString *string = [[NSString alloc] initWithFormat:#"test"];. That is, it determines the literal may be reused, and reuses it in this context. After all, the input matches the output.
However, you should not rely on these internal optimizations in your programs -- just stick with the reference counting rules and well-defined behavior.
Update
David's comment caused me to look into this. On the system I tested, NSString *string = [[NSString alloc] initWithFormat:#"test"]; returns a new object. Your program messages an object which should have been released, and is not eligible for the immortal string status.
Your program still falls into undefined territory, and happens to appear to give the correct results in some cases only as an artifact of implementation details -- or just purely coincidence. As David pointed out, adding 'stuff' between the release and the log can cause string to really be destroyed and potentially reused. If you really want to know why this all works, you could read the objc runtime sources or crawl through the runtime's assembly as it executes. Some of it may have an explanation (runtime implementation details), and some of it is purely coincidence.
Doing things to a released object is an undefined behavior. Meaning - sometimes you get away with it, sometimes it crashes, sometimes it crashes a minute later in a completely different spot, sometimes a variable ten files away gets mysteriously modified.
To catch those issues, use the NSZombie technique. Look it up. That, and some coding discipline.
This time, you got away because the freed up memory hasn't been overwritten by anything yet. The memory that string points at still contains the bytes of a string object with the right length. Some time later, something else will be there, or the memory address won't be valid anymore. And there's no telling when this happens.
Sending messages to nil objects is, however, legitimate. That's a defined behavior in Objective C, in fact - nothing happens, 0 or nil is returned.
Update:
Ok. I'm tired and didn't read your question carefully enough.
The reason you are not crashing is pure luck. At first I though that you were using initWithString: in which case all the answers (including my original one (below)) about string literals would be valid.
What I mean by "pure luck"
The reason this works is just that the object is released but your pointer still points to where it used to be and the memory is not overwritten before you read it again. So when you access the variable you read from the untouched memory which means that you get a valid object back. Doing the above is VERY dangerous and will eventually cause a crash in the future!
If you start creating more object in between the release and the log then there is a chance that one of them will use the same memory as your string had and then you would crash when trying to read the old memory.
It is even so fragile that calling log twice in a row will cause a crash.
Original answer:
String literals never get released!
Take a look at my answer for this question for a description of why this is.
This answer also has a good explanation.
One possible explanation: You're superfluously dynamically allocating a string instead of just using the constant. Probably Cocoa already knows that's just a waste of memory (if you're not creating a mutable string), so it maybe releases the allocated object and returns the constant string instead. And on a constant string, release and retain have no effect.
To prove this, it's worth comparing the returned pointer to the constant string itself:
int main()
{
NSString *s = #"Hello World!";
NSString *t = [[NSString alloc] initWithFormat:s];
if (s == t)
NSLog(#"Strings are the same");
else
NSLog(#"Not the same; another instance was allocated");
return 0;
}